Millimeter Waves (mmWaves) are radio waves with wavelength in the range of 1 millimeter (mm)-10 mm, which corresponds to a radio frequency of 30 GigaHertz (GHz)-300 GHz. Per the definition by the International Telecommunications Union (ITU), these frequencies are also referred to as the Extremely High Frequency (EHF) band.
The mmWaves exhibit unique propagation characteristics. For example, compared with lower frequency radio waves, mmWaves suffer higher propagation loss, have a poorer ability to penetrate objects, such as buildings, walls, foliage, and are more susceptible to atmosphere absorption, deflection and diffraction due to particles (e.g., rain drops) in the air. On the other hand, due to the smaller wavelengths of the mmWaves, more antennas may be packed in a relatively small area, thereby allowing for the implementation of a high-gain antenna in small form factor.
The mmWaves have been less utilized than the lower frequency radio waves. A vast amount of spectrum is available in the mmWave band. For example, the frequencies around 60 GHz, which are typically referred to as the 60 GHz band, are available as unlicensed spectrum in most countries.
In wireless communications, channel state information (CSI) refers to known channel properties of a communication link. This information describes how a signal propagates from the transmitter to the receiver and represents the combined effect of, for example, scattering, fading, and power decay with distance. The CSI makes it possible to adapt transmissions to current channel conditions for achieving reliable communication with high data rates in multiantenna systems. The CSI needs to be estimated at the receiver, but can be quantized and fed back to the transmitter (although reverse-link estimation is possible). The transmitter and receiver can have different CSI.
Accurate CSI estimation is desirable in order to take advantage of the potential of mmWave network capacity. In particular, with accurate CSI estimation, CSI can be exploited for optimum or quasi-optimum precoder and equalizer designs with the purpose of maximizing system performance. A number of different methods are developed for estimating CSI. However, the CSI estimation methods developed for lower frequency radio waves are not always suitable for mmWaves channel estimation due to differences in the physical characteristics of the waves.
One method, described by Roi Mendez-Rial in “Channel Estimation and Hybrid Combining for mmWave: Phase Shifters or Switches,” estimates the mmWave channel using the sparse recovery formulation of the channel. However, such a formulation does not accurately represent different properties of the mmWave channel, which makes this method less accurate.